PHP: Almost manually testing add()

In the first examples we recreate a path similar to what we
took in the Perl introduction so you might skip it or just
flip through the pages.

Let's assume we have a simple PHP library with a bunch of
functions. It is located in our includes directory in the
mylib.php file. It is used by a "web application" called
basic_calc.php that provides a - surprise - calculator
for the web visitor.

In order to test this we create a separate PHP script that
will require the relevant library and call the add()
function supplying various arguments and displaying the results.